Electromagnetic radiation absorbing materials typically comprise one or more kinds of dissipative particles dispersed through a dielectric binder material. For example, U.S. Pat. No. 4,173,018 (Dawson et al.) discloses a material comprising an insulating resin and solid iron spheres of 3 microns diameter, or solid glass spheres of 0.4 micron diameter having a single 1.3 micron thick iron coating, for a total diameter of 3 microns. The particles comprise up to 90% of the weight of the composite material.
Substantially spherical solid particles of such sizes are often called "microspheres." A variation on the microsphere is the "microbubble," a hollow microsphere made of a material such as glass. Single thin film layers of nonmagnetic metal may be deposited on glass microbubbles, and the product dispersed through polymeric binders, as taught in U.S. Pat. No. 4,618,525 (Chamberlain et al.)
Singly layered microbubbles dispersed through polymeric binders have been used in electromagnetic shielding applications. For example, U.S. Pat. No. 4,624,798 (Gindrup et al.) describes a composite material in which the microbubbles form a network of contacting particles, giving the bulk material sufficient electrical conductivity to act as a radiation shield, i.e., like a sheet of conductive material.